The provision of steam to a turbine is typically accompanied by the transfer of heat between the steam and those parts of the turbine coming into contact with the steam. This heat transfer has the tendency to create thermal distortion in various components of the turbine due primarily to thermal expansion and/or contraction occurring as a result of the heat transfer. Any resulting deformation of such turbine parts can be of two types: elastic which is recoverable upon release of the distortion, and plastic which is permanent. In certain applications plastic deformation can be significant enough to damage the horizontal joint flange of the turbine inner cylinders, requiring costly repairs and replacement of damaged parts.
Thermal distortion could also become significant enough to cause an ovalized deformation of the ends of the turbine inner cylinder. Such ovalized deformation can cause portions of the inner cylinder which are in close proximity to the rotor blades to move away from such blades resulting in increased clearance and attendant leakage. A far more serious consequence of such ovalized deformation is where portions of the inner cylinder move toward the rotor blades. If such movement is significant, the rotor blades will rub the surface of the inner cylinder causing damage and degradation of efficiency.
It is therefore desirable to minimize the effects of thermal deformation in order to preserve the efficiency as well as the proper alignment of the turbine.
The problems caused by thermal deformation of turbine components are a particularly important consideration in the design of low pressure steam turbines. In low pressure steam turbines there is a significant difference in the steam temperature at the turbine inlet and at the turbine exhaust or annulus. For example, it has been determined that steam entering a low pressure turbine inlet can have a temperature of approximately 700.degree. F., whereas the temperature of the steam as it crosses the last row of blades can be approximately 100.degree. F. The thermal loading resulting from such a temperature drop can cause the above described effects.
In a low pressure steam turbine, undesirable flange stiffness in the support area of the inner cylinder can contribute significantly to cause large thermal bending moments in the horizontal joint flange. Since the horizontal joint flange represents a structural component which is integral to the support of the inner cylinders, any excessive bending moments will cause thermal deformation of the semi-cylindrical cross-sectional shape of the cylinder casings or rings, causing them to deform out-of-round, i.e., the above mentioned phenomena of ovality.
As pointed out in U.S. Pat. No. 4,863,341--Groenendaal, Jr., it has been found that increasing the flexibility of the horizontal joint flange greatly reduces thermal deformation in the cylinder rings, thus minimizing the loss of efficiency due to ovality. As disclosed in the above referenced patent, a more flexible horizontal joint flange has been achieved by strategically isolating the inlet chamber of the inner cylinder, and further, by eliminating axial members between rings in the plane of the horizontal joint flange.
Although the structure described in the above referenced patent is capable of reducing the stiffness of the horizontal joint flange, only a 70% reduction of thermal deformation in the cylinder rings has been realized. Since further reduction of the thermal deformation in the cylinder rings is desired in order to achieve maximum efficiency of the turbine and operating costs savings, there remains a significant need for low cost-low maintenance technology advances in this area.
Also, in those cases where turbine operating budgets do not provide the resources necessary for replacement turbines in order to increase efficiency, the only viable alternative to keep a turbine in operation is a modification or retrofit of the existing low-pressure turbine. Although the prior art does provide steps to retrofit turbines in varying degrees, such as U.S. Pat. No. 4,900,223--Groenendaal, Jr., modification technology is necessary to increase the flexibility of the horizontal joint flange in order to increase the efficiency of the existing turbine and keep the turbine in operation.